Exploring polymer dielectrics for donor-acceptor and small molecule based transistors and voltage inverters

Abstract

Organic semiconductor devices have gained much attention over the past several decades. Solution processability of polymer materials lends itself towards the realization of flexible, wearable electronic devices, owing to the low energy needs and the applicability of large-scale roll-to-roll casting methods. Unlike traditional field effect transistor technologies which rely on doped semiconductors and operate in the inversion regime, organic transistor devices utilize intrinsic semiconductors and operate in the accumulation regime. For this reason, the unique physical properties of both the dielectric layer and the semiconductor layer are crucial to driving charge transport. Hole-transporting field effect transistors were fabricated using the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) as the dielectric along with a donor-acceptor polymer based on diketopyrrolopyrrole as the semiconductor layer. These devices yield carrier mobilities upwards of 0.4 cm2/Vs and high on/off ratios when the ferroelectric layer is electrically poled. Furthermore, through the application of self-assembled monolayers, the device characteristics are observed to be modified by orders of magnitude for devices of the same architecture utilizing a non-ferroelectric dielectric. Fluorination of donor-acceptor copolymers has been one strategy employed towards enhancing polymer coplanarity, increasing crystallinity, and improving charge transport mechanisms in organic devices. Towards the realization of dedicated n-type polymer semiconductors, the synthesis of thiazole flanked fluorinated isoindigo copolymers is reported and their field effect transistor properties are demonstrated. The selenophene-substituted isoindigo shows improved performance over the thiophene units. Top-gate devices with varying dielectric layers showed n-type transport with electron carrier mobilities of the order of 10-2 cm2/Vs and on/off ratio of 105. Several proof-of-concept complementary voltage inverter circuits have been constructed utilizing the various dielectrics and donor-acceptor semiconductors explored prior in this work. In addition to these, ozone-treated zinc oxide and the donor-acceptor polymer quinoxaline are explored for application in constructing inverter circuits. The contribution of each device's characteristic to the overall inverter behavior is demonstrated. Furthermore, a basic inverter model is used to simulate the inverter characteristics of one such circuit using parameters from the individual device characteristics. A viable route for enhancing the dielectric constant of polymer dielectrics is via the incorporation of semiconducting and insulating nanoparticles. Returning the focus back to the influence of the dielectric layer, improved performance of organic capacitor and transistor devices after incorporating magnetic nanoparticles into non-ferroelectric dielectrics is observed. In particular, the threshold voltage, subthreshold swing, and observable hysteresis are reduced in pentacene devices using cross-linked poly(4-vinyl phenol) polymer dielectric with incorporated cobalt ferrite nanoparticles. The application of an external magnetic field is also observed to further tune device behavior.